Analyzers using plasma torches have been used for many years for the analysis of components contained in liquid samples. Typically the liquid sample is sprayed in a spray chamber, using pneumatic nebulization, to form a fine mist of droplets. The fine droplets from the mist, and the fine particles which remain when droplets are evaporated, are introduced into the plasma torch where they are vaporized and ionized. Analysis is typically performed by connecting a mass spectrometer or other mass analyzer to the torch to receive ions from the torch, or by spectroscopy, i.e. by optically analyzing light emitted from the plasma.
In apparatus of the kind described, proper design of the sprayer and spray chamber are important to achieve optimum results. Poor design can result in low signal, or an unduly long signal rise time when spraying begins, or an unduly long washout time to clean out the spray chamber before a new sample can be introduced. In addition, some spray chambers waste a high proportion of the sample provided to them.
One example of an apparatus used for providing liquid sample to a plasma torch is shown in U.S. Pat. No. 5,345,079 to John B. French and Bernard Etkin, two of the present inventors. However this device requires the sample be directed in a stream of uniformly sized and spaced droplets. This can in some cases be a more complex and less convenient procedure than simply spraying a nebulized sample, for example as shown in U.S. Pat. No. 4,861,988.
U.S. Pat. No. 5,170,052 shows a method of using nebulizing gas to form a mist from a sample liquid and to inject the mist into a corona discharge. The technique shown in this patent involves heating the gas which is used to nebulize the liquid, an undesirable procedure which can result in breakdown of the molecules to be analyzed and which can also lead to clogging of fine orifices.
U.S. Pat. No. 5,477,048 shows a conventional form of nebulizer in which coarse droplets are sorted by momentum and wasted to a drain, while fine droplets which are able to negotiate a sharp turn are directed to a plasma torch. This approach has the disadvantages of wasting a great deal of sample and producing a relatively low signal. It also can produce severe memory effects and therefore requires lengthy and thorough washout before a new sample solution can be introduced.
Therefore, it is an object of the present invention to provide a new spray chamber and method, in which signal levels can be improved and in which signal rise time, washout time and memory effects may all be reduced. The new spray chamber and method may advantageously be used not only with analyzers which use plasma torches, but also with other kinds of analyzers, e.g. mass analyzers which use atmospheric pressure ionization. The present invention in another aspect relates to the use of the new spray chamber with a dryer.